The present invention relates generally to integrated circuit assembly testing, and more particularly to a method for non-contact testing of fixed and inaccessible connections without using a capacitive sensor plate.
Capacitive probe testing is frequently used in determining the integrity of integrated circuit connections. Capacitive probe testing utilizes non-contact capacitive coupling technologies, for example those described in U.S. Pat. No. 5,557,209 to Crook et al., entitled “Identification Of Pin-Open Faults By Capacitive Coupling Through The Integrated Circuit Package”, U.S. Pat. No. 5,420,500 to Kerschner, entitled “Capacitive Electrode System For Detecting Open Solder Joints In Printed Circuit Assemblies”, and U.S. Pat. No. 5,498,964 to Kerschner et al., entitled “Capacitive Electrode System For Detecting Open Solder Joints In Printed Circuit Assemblies”, all of which are hereby incorporated by reference for all that they teach.
Capacitive probe testing has not traditionally been used to test fixed pins or tied pins because of the lack of diagnostic separability and presence of significant capacitance due to board-mounted bypass capacitors. A fixed pin is usually considered to be a power or ground pin because it cannot be moved easily with a test stimulus. A tied pin is considered to be any pin for which several other pins on the same device (such as an integrated circuit or connector) share the same node. Note that because devices such as integrated circuits and connectors typically provide multiple power and ground pins, the power and ground fixed pins may also be tied pins as well. For purposes of this patent, the terms “fixed” and “tied” will be used interchangeably because the differences in terms of the present invention are slight.
Recent patent applications U.S. patent application Ser. No. 10/703,944, entitled “Methods and Apparatus For Testing And Diagnosing Open Connections For Sockets And Connectors On Printed Circuit Boards” to Parker et al, and U.S. patent application Ser. No. 10/836,862, entitled “Methods and Apparatus For Non-Contact Testing And Diagnosing Open Connections For Connectors On Printed Circuit Boards” to Parker et al., each of which is incorporated by reference for all that it teaches, collectively describe a method for testing opens on fixed/tied pins on connectors. The method described tests the connector fixed pins implicitly by analyzing inherent capacitive structures present in the network. FIG. 1 is a schematic diagram of a test setup illustrating the prior art non-contact techniques described. As illustrated, the test setup 10 includes a tester 20, a board 30 being tested by the tester 20, a device 40 on the board, and a capacitive sense plate 50 and amplifier circuitry 60 of a capacitive sensor device. In operation, an AC signal 22 is applied to a signal pin 41 which is capacitively coupled (Cc) to the fixed pin 43 which is being implicitly tested via a capacitor 42 inherently formed between the pins 41, 43. A sense plate 50 is positioned above the device 40, forming capacitors (Cs) 52, 54 between the sense plate 50 and pins 41, 43 of the device 40. Nearby amplifier circuitry 62 is used to sense, amplify, and filter noise from an AC current signal Is related to an effective capacitance Ceff between the sense plate 50 and device 40. Hardware and software in the tester 20 convert the measured AC current signal Is into a capacitance value Ceff.
If no defect is present at pin 43 with connection joint 32 (i.e., no open exists), the tester 20 will simply measure the capacitance value CS due to capacitor 52. If an open defect is present at connection joint 32, a capacitance Copen 34 is introduced between the device and the board to which it is soldered. This, combined with capacitance Cs provided by capacitor 54, results in an effective capacitance Ceff at the sense plate of (Cs+Δ). The diagnostic software in the tester 20 recognizes this higher reading as a defect.
This concept is extended to not only include fixed open pins, but also inaccessible shorted pins by U.S. patent application Ser. No. 10040181, to Parker, entitled “Methods And Apparatus For Non-Contact Testing And Diagnosing Of Inaccessible Shorted Connections”. Inaccessible pins are considered to be pins for which the tester does not have probe access and therefore cannot stimulate with an AC source. The coverage of inaccessible pins is not limited to connectors but can also include integrated circuit devices.
The prior art capacitive probing testing techniques mentioned above all rely upon an external sense plate and external amplification and signal conditioning hardware in the tester's fixture to obtain the capacitive measurements. This hardware can be costly in terms of parts, engineering time, and installation time. Installation of the hardware into the tester fixture can also add debug time due to problems when the sense plate is not properly aligned over the device under test, when there are poor connections, when the hardware is installed incorrectly, or when the hardware is defective.
In addition, the ability of the tester software to diagnose a defect is dependent on the measurement accuracy of the readings. As mentioned, when implicitly testing the capacitively coupled pin, the effective capacitance on the signal pin measures Cs when good and Cs+Δ when defective. The accuracy of the tester must be good enough to reliably discriminate between these two values given their measurement uncertainties. Measurement uncertainty is a natural byproduct of any measurement system. On many successive measurements of the same value, a range of readings will be observed that will form a bell curve peaked near the actual value of the property being measured. FIG. 2 is a graph illustrating capacitance measurement bell curves for a signal pin with a good (or NoDefect) connection (Cs) (curve 81), and the same signal pin with a defective (or Defect) connection (Cs+Δ) (curve 82), as measured on a tester with small measurement uncertainty. As shown, the tester with small measurement uncertainty can easily discriminate between the NoDefect connection and the Defect connection.
FIG. 3 is a graph illustrating capacitance measurement bell curves for a signal pin with a good (or NoDefect) connection (Cs) (curve 83), and the same signal pin with a defective (or Defect) connection (Cs+Δ) (curve 84), as measured on a tester with higher measurement uncertainty. As shown, the tester with higher measurement uncertainty cannot reliably discriminate between the NoDefect connection and the Defect connection. With the given measurement accuracy of the tester used to make the measurements in FIG. 3, there is some overlap 85 of the bell curves. Any reading that occurs in the overlap area 85 is ambiguous as to indicating a defect. The only solution for obtaining a reliable test coverage is to increase the difference between the Defect and NoDefect readings, which will separate the two bell curves 86 and 87 in FIG. 4. The same tester that had reliability problems in FIG. 3 does not have the reliability problems in FIG. 4 because of a larger Δ. Unfortunately, Δ is not a modifiable parameter for the tester. Rather, Δ is a function of Cs and Cc in FIG. 1, which are device characteristics.
Accordingly, it would be desirable to have a technique which allows an increased Δ to avoid ambiguity in capacitive measurement readings. It would also be desirable to have a technique for a non-contact measurement system that does not require a sensor plate to make the capacitive measurements.